Internal and External Forces Consider a system E.g. a baseball All atoms/particles inside interact with each other Atom 1 acts on atom 2 with F21 But atom 2 also acts on atom 1 with F12 Newton's 3rd law says that F12=-F21 So the net force is zero . same for all pairs of particles All interanl forces add up to zero Only external forces remains E.g., gravity or the contact of a stick We will deal with external forces mostly Physics 121: Lecture 3, Pg 10Physics 121: Lecture 3, Pg 10 Internal and External Forces Consider a system E.g. a baseball All atoms/particles inside interact with each other Atom 1 acts on atom 2 with F21 But atom 2 also acts on atom 1 with F12 Newton’s 3rd law says that F12 = - F21 So the net force is zero … same for all pairs of particles All interanl forces add up to zero Only external forces remains E.g., gravity or the contact of a stick ! We will deal with external forces mostly